<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style type="text/css">
.wrap { background: yellow; width: 200px; height: 200px; position: relative; }
.center { background: green; position: absolute; width: 100px; height: 100px; left: 50%; top: 50%; margin-left: -50px; margin-top: -50px; }
</style>
</head>

<body>
<div class="wrap">
  <div class="center"></div>
</div>
</body>
<script>
function foo() {
  var x = 1;
  return function () { alert(x); };
};
 
var bar = foo();
 
bar(); // 1
 
eval('x = 2', bar); // 传入上下文，影响了内部的var x 变量
 
bar(); // 2

eval('var x = 10');
 
(function foo() {
  eval('var y = 20');
})();
 
alert(x); // 10
alert(y); // "y" 提示没有声明
</script>
</html>
